There are known methods which rely on fermentation for preparing L-malic acid. A microorganism having fumarase activity is cultivated in a culture medium containing fumaric acid and L-malic acid is collected from the product of cultivation. See, for example, Japanese Patent Publication No. 16547/66. These methods, however, require a large apparatus for fermentation and yet have a very low yield of L-malic acid. The acid prepared by these methods is, therefore, expensive.
There are also known methods which employ an enzyme. The fumarase in a microorganism having fumarase activity is used to prepare L-malic acid from fumaric acid or a salt thereof. See, for example, The Journal of General and Applied Microbiology, 6, 108-116 (1960), European Journal of Applied Microbiology, 3, 169-183 (1976) and Japanese Patent Publication Nos. 4511/62 and 1191/69. The preparation of L-malic acid on an industrial scale by these methods requires the use of a microorganism having a very high fumarase activity. It is, however, very difficult to separate any such microorganism from the natural world. This has been a great obstacle to the application of these methods on an industrial basis.
Both of these known methods also have a common problem which is due to the use of a microorganism. Succinic acid is often produced from fumaric acid as a by-product and is difficult to separate from L-malic acid efficiently. It is, therefore, important to prevent the formation of succinic acid as a by-product in order to prepare L-malic acid effectively on an industrial basis.
There are known a couple of methods for preventing the formation of succinic acid. According to one of them, a microorganism is immobilized and brought into contact with an organic solvent (Japanese Patent Publication No. 8396/77). According to the other method, an immobilized microorganism is brought into contact with bile acid (Japanese Patent Publication No. 31952/77). The use of an organic solvent, however, calls for special care from a safety standpoint and, therefore, special equipment. Bile acid is expensive. Moreover, it requires a long time for completing the necessary reaction. Therefore, both of these methods have been unsatisfactory from the standpoint of industrial application and there has been a strong demand for a simpler and less expensive method.